As a semiconductor laser module having the function of controlling the temperature of a semiconductor laser, there is a known unit described in FLD5Fl4CN-E19 to E58 of FUJITSU SEMICONDUCTOR DEVICE DATA SHEET (DS02-12601-2). The automatic temperature control circuit described in the DATA SHEET has a Peltier element and a thermistor contained in a laser diode and an analog integrator and used for temperature control by proportion integration (P. I) control so that the actual temperature conforms to a set temperature.
Absolute precision and absolute stability of wavelength are required in the wavelength division multiplexing (WDM) communication system. Although distribution feedback (DFB) type semiconductor lasers are employed in such a WDM communication system, the temperature of the laser at the time of oscillation needs controlling precisely because the oscillation wavelength of the DFB type laser is easily affected by the temperature of the laser. The oscillation wavelength between adjoining channels tends to be set at 0.8 nm or less in the WDM communication system in recent years. The required wavelength precision of the laser for use in the system like this is 0.01 nm. In order to attain these requirements, it is needed to control the temperature of the laser at the time of oscillation within 0.1° C.
In the above-described prior-art automatic temperature control circuit, a steady state may be brought about in a state that the actual temperature deviates from a set temperature under the influence of thermal resistance and the like. In order to increase the precision of the temperature control by reducing the deviation between the actual temperature and the set temperature, the loop gain of a control system needs increasing. In the automatic temperature control circuit having a closed loop using the Peltier element, however, though it is possible to freely set a temperature in the case of PI control through digital processing, there occurs a great delay in transmission besides the primary delay between the Peltier element and the thermistor in the case of analog processing as the system is generally such that the response of the temperature is extremely slow in comparison with the response of the electronic circuit. For this reason, oscillation is likely to arise when the loop gain of the control system is increased too much. Therefore, the loop gain cannot be increased immoderately and this makes it difficult to raise the precision of the temperature control.
An object of the present invention is to provide a semiconductor laser module for making precise temperature control possible.